1. Field of the Invention
The present invention relates generally to nuclear reactor power plants or facilities, and more particularly to a remotely adjustable intermediate seismic support system which is adapted to be mounted upon the control rod drive mechanism (CRDM) latching housings so as to prevent excessive deflections of the CRDM rod travel housings under high seismic force load conditions which would otherwise or conventionally result in excessive stress loads being impressed upon the CRDM head penetration tube-reactor vessel closure head penetration welds, as well as adversely affecting the scrammability operations of the power plant or facility.
2. Description of the Prior Art
In a pressurized water reactor (PWR), the control rod drive mechanisms (CRDM), as well as the displacer rod drive mechanisms (DRDM) which control the influx or displacement of the moderator water within the reactor core, extend vertically upwardly in a cantilevered fashion from the nuclear reactor pressure vessel closure head. The lower ends of the CRDM and DRDM head penetration tubes are disposed within bores, defined within the closure head, by means of shrink fittings, and the lower end portions of the head penetration tubes are peripherally welded to the undersurface of the closure head. When the power plant or facility is then subsequently placed in operation, however, the temperature level of the atmosphere within the reactor pressure vessel is substantially increased, and consequently, it has been observed that those portions of the closure head which define the CRDM and DRDM head penetration tube bores expand radially at a rate which is greater than the radial expansion rate of the CRDM and DRDM head penetration tubes per se. Consequently, the lower ends of the CRDM and DRDM head penetration tubes are no longer supported within the reactor pressure vessel closure head by means of both the shrink fittings and the undersurface peripheral weldments, but solely by means of the weldments. It is therefore to be appreciated that such welds cannot withstand excessive stress loading due to the relatively small extent of the load bearing surfaces such weldments present to the substantial torque and bending moments to which the welds will be subjected.
When the reactor is thus subjected, for example, to seismic loads, the long rod travel housings of the CRDM and DRDM systems tend to undergo substantial deflection from their normal dispositions which, in turn, impress excessive stress loads upon the CRDM head penetration tube-pressure vessel closure head welds. In addition, should such deflection loads impressed upon the CRDM rod travel housings be especially severe as to deform the rod travel housings, the scram capabilities of the CRDM drive rods, and the control rods actuated thereby, will be adversely affected. In order to counteract these operatively deleterious effects, conventional power plants fix the upper free ends of the CRDM and DRDM rod travel housings through means of tie rods which extend between the plant or facility building walls and an upper seismic support ring which peripherally surrounds rod position indicator (RPI) coil assembly upper plates which are respectively disposed at the uppermost extent of each RPI coil assembly so as to surround the upper free end of each CRDM and DRDM rod travel housing. Such means may in fact prove substantially satisfactory for those power plants or facilities which may be located within regions that may be subjected to relatively low-level or intermediate level seismic forces in that the stress loads and adverse scrammability effects can nevertheless be maintained within acceptable limits, however, under high-level seismic loading conditions, such fixture means will prove to be entirely inadequate for achieving the desired and required restraint functions, especially in connection with the CRDM rod travel housings. More particularly, while such fixture means operatively associated with the upper free ends of the CRDM and DRDM rod travel housings may prove to be entirely satisfactory and adequate in sufficiently restraining deflection movements of the DRDM rod travel housings, even under high-level seismic loading conditions, in view of the fact that such DRDM rod travel housings do not constitute any structure which exhibits any substantial mass concentrations, the contrary, of course, is true in connection with the CRDM rod travel housings. Specifically, a substantial concentration of mass is defined within the CRDM latch housing and coil stack assemblage. Consequently, under high-level seismic loading conditions, despite the fact that the upper and lower ends of the CRDM rod travel housings are fixed, the axially central or intermediate portion thereof will undergo substantial deflection movements with manifestation of the aforenoted adverse stress loading and scrammability effects. A need therefore persisted within the prior art for a suitable means for effectively restraining the axially central or intermediate portion of each CRDM rod travel housing, particularly under high-level seismic loading conditions.
Other prior art or conventional pressurized water reactors (PWRs) did in fact seek to satisfy the aforenoted need for such an intermediate seismic support system for use in conjunction with the CRDM rod travel housings, and thereby provide a solution to the continuing and omnipresent problems of excessive CRDM rod travel housing deflections under seismic loading conditions, by the provision of a suitable intermediate seismic support structure which operatively engaged, or was operatively secured to, the CRDM rod travel housings at locations intermediate the reactor pressure vessel closure head and the upper fixed ends of the CRDM rod travel housings, however, such intermediate seismic support structures require access to be provided for the maintenance personnel in order to accomplish the required installation, adjustment, and removal operations. This is not particularly desirable in view of the fact that once the particular reactor has become operational, the region within the vicinity of the reactor pressure vessel closure head, within which the personnel would be required to work, is a moderate radiation zone. In addition, personnel access simply cannot be provided in connection with particular types of pressurized water reactors. For example, the core of one particular type of pressurized water reactor contemplated for commercial power plant usage by the assignee of the present patent application has incorporated therewithin one-hundred eighty-five (185) drive mechanisms, in particular, ninety-seven (97) control rod drive mechanisms (CRDMs) and eighty-eight (88) displacer rod drive mechanisms (DRDMs), all of which are relatively spaced on 9.996 inch center-to-center distances. Consequently, access for maintenance personnel within the vicinity of the intermediate seismic support structure cannot in fact be accommodated.
Accordingly, it is an object of the present invention to provide a new and improved intermediate seismic support structural system.
Another object of the present invention is to provide a new and improved intermediate seismic support structural system which resolves the aforenoted problems characteristic of prior art seismic support systems.
Yet another object of the present invention is to provide a new and improved intermediate seismic support structural system which will effectively prevent excessive deflections of the CRDM rod travel housings relative to the nuclear reactor pressure vessel closure head even under high-level seismic loading conditions.
Still another object of the present invention is to provide a new and improved intermediate seismic support structural system which will eliminate the requirement of providing for, or accommodating, access to such support structure in order to accomplish installation, maintenance, or removal operations.
Yet still another object of the present invention is to provide a new and improved intermediate seismic support structural system which is capable of being remotely installed, adjusted, and removed as required.
Still yet another object of the present invention is to provide a new and improved intermediate seismic support structural system which will effectively prevent adverse load conditions from being impressed upon the CRDM and DRDM penetration tube-reactor pressure vessel closure head welds, even under severe seismic loading conditions.
A further object of the present invention is to provide a new and improved intermediate seismic support structural system which will effectively prevent adverse load conditions from being impressed upon the CRDM and DRDM rod travel housings, even under severe seismic loading conditions, so as not to adversely affect the scram capabilities of the CRDM drive rod-control rod systems.
A yet further object of the present invention is to provide a new and improved intermediate seismic support structural system which can operationally interface with existing nuclear reactor pressure vessel closure head structural components or equipment, such as, for example, the CRDM coil stack assembly, the CRDM latch housing, the CRDM coil stack lead tube, the CRDM and DRDM rod travel housings, the CRDM and DRDM rod position indicator (RPI) coil assemblies, and the like, without interfering with the normal operations thereof.
A still further object of the present invention is to provide a new and improved intermediate seismic support structural system which comprises intermediate seismic support elements or components respectively mounted upon the CRDM systems in an individual or separate manner, wherein, however, the seismic support elements may be integrated together so as to provide enhanced support for the CRDM systems against lateral or transverse deflection loading.